In the use of turbines for the production of electrical energy by the use of high pressure gases, which are typically high pressure, high temperature combustion gases, to drive the turbines, the turbine blades are typically configured to a clearance (gap) of about 0.20 inches between the outer ends, or tips, of the turbine blades and the inside of the turbine casing, or a shroud or block positioned inside the turbine casing, in an annular position in the same plane as the rotation of the turbine blades. This clearance is provided to avoid any contact of the blade tips with the inside of the casing or the shroud.
Many turbine blades are designed so that cooling air flows outwardly through the turbine blades from a turbine blade wheel and is discharged from the turbine blade tips. In other turbine blades, the cooling gas, which may be steam, air or the like, is circulated inside the blade and may be discharged from a position other than the turbine blade tips, such as the back side of the turbine blades and the like.
The shroud blocks, as discussed, are circumferentially positioned inside the casing in a common plane with the turbine blades and may be hollow and air-cooled. Modern shroud blocks are coated on their inside surface facing the blade tips with special abradable coatings. The purpose of these coatings is to minimize the loss of material at the tip of the blades during abnormal, transient conditions such as sudden variations in turbine workload, start-up, shutdown and the like. In extreme cases where the turbine blades are rubbing the shroud blocks to the extent of removing the abradable coatings, coating shavings may be trapped in the blade cooling holes. As a result, the blade may begin to overheat. The reduced amount of cooling air being issued at the tip of each blade will cause heating of the gases leaking through the gaps. In the instance where the tips of the blades may be rubbed off or the blades are not coaxially positioned inside the casing or the like so that the gap between the tips of the blades and the inside of the casing or the shroud is increased, additional amounts of hot gas will spill over the blade tips. This hot gas is basically wasted for energy generation.
Accordingly, whenever the blade tips are rubbing the abradable coating, higher temperatures may be generated because of the friction between the blade and the abradable coating or as a result of plugging of air cooling channels in the blades and the like. This condition will likely result in the presence of a larger than desired gap on the opposite side of the turbine so that hot gases spill over the tips of the blades and through the gap with the resulting wasted energy. Neither of these conditions is desirable but both are difficult to detect in an operating turbine.
Accordingly, a continuing effort has been directed to the development of methods to determine the performance of operating turbines.